Two U.S. Border Patrol agents were assaulted Sunday and one died of his injuries, an agency union official now says.

Rogelio Martinez and his partner were responding to a triggered ground sensor off Interstate 10 near the U.S.-Mexico border, Sunday morning. Martinez’s partner then called for help. Martinez died in the hospital. He was 36 and a native of El Paso. He’d served with the Border Patrol a little more than four years.

"It appeared that due to the injuries that were seen on the agents, it appeared that they had been assaulted," said del Cueto.

Del Cueto said Martinez’s partner suffered severe head trauma.

President Donald Trump said Martinez’s killers would be brought to justice and renewed calls for a border wall.

Texas Gov. Greg Abbott offered a $20,000 reward for information on what he called Martinez’s murder. Arizona Republican Martha McSally called his death senseless and said on Twitter it should serve as a wake-up call to the country.

"There’s a sense of frustration. There’s a sense of anger and there’s a sense of sadness," del Cueto said.

The FBI is investigating the incident and has said only that Martinez did not sustain a gunshot wound.

According to the Border Patrol's use of force statistics, assaults on agents in the last fiscal year were well above those tallied from the previous year. 671 assaults were reported through September 2017 compared to 403 in fiscal year 2015.

Last summer, an off-duty Border Patrol agent from the Deming, New Mexico, station was assaulted and suffered injuries to his head, chest and hands. The victim was found on the side of a road by a passing motorist at 11 p.m.